This invention relates to a telecommunications system with a call waiting feature, and more particularly a telecommunications system where a plurality of subscribers are connected to a central office in such a way that a tone is transmitted over an open line to alert an active subscriber to the presence of a second incoming call.
Call waiting has been offered by most telephone companies for a number of years. If, for example, the subscriber is awaiting an urgent incoming call he or she can continue to use the telephone without fear of losing the important call. On receipt of the call waiting tone, the subscriber can terminate the existing call in progress and speak to the calling party, or alternatively place the previous call on hold and switch between the two calls.
Call waiting is a very useful feature in the voice mode, but unfortunately the transmitted tone interferes with modem connections in the data transmission mode. Most companies provide a cancellation feature which allows the subscriber to enter a predetermined (Cancel Call Waiting) CCW access code and disable the call waiting feature.
Call waiting can be disabled either at the start of a call by entering the appropriate access code or during a call by flashing and dialing the access code. The CCW feature is automatically cancelled at the termination of the call.
The CCW feature works satisfactorily when the subscriber dials up a remote computer and initiates modem transmission. However, as a security feature many host computers operate in the dial-up/dial-back mode. In this mode, the subscriber dials up the host computer to enter the computer system and his personal identification code on the telephone key pad. He then hangs up. Subsequently, the host computer dials back the subscriber and establishes the modem connection. Unfortunately, in this situation the standard CCW feature does not work because when the subscriber hangs up his phone, the CCW feature is automatically cancelled. After the subscriber's modem has answered the incoming call, it is not feasible to flash and enter the CCW code because the modem connection is immediately broken.